Electrochemical nitrate reduction reaction (NO₃RR) shows great promise for the recycling of nitrate from wastewater sources for the denitrification of wastewater and sustainable ammonia production. Among various electrocatalytic materials, single-atom catalysts such as Fe−N−C have received considerable attention for the NO₃RR, due to the well-defined Fe−N₄ active sites and finely tailorable coordination environment. Here, we investigate the pH-dependent mechanism of NO₃RR on Fe−N−C single-atom catalyst by combining experiments and DFT calculations. The Fe−N−C catalyst is prepared by pyrolysis of a metal-organic framework (MOF) precursor and then characterized by multiple techniques to confirm its chemical and atomic structure. The Fe−N−C catalyst exhibited high activity and >80% Faradaic efficiency for NH₃ production in acidic, neutral, and alkaline electrolytes, showing its outstanding performance for the NO₃RR across all electrolyte conditions. Interestingly, there was a negligible production of NO₂⁻ in the alkaline electrolyte, but HER exhibited the strongest competition with NO₃RR in the alkaline electrolyte, rather than in the acidic electrolyte. DFT calculations identified the NHO*-mediated pathway as the most favorable pathway for NO₃RR to NH₃ on Fe−N−C and revealed the pH dependence of the rate-determining steps of both NO₃RR and HER, elucidating the selectivity observed under different pH conditions. Our studies provide a deep understanding of the NO₃RR mechanism on single-atom catalysts as well as new insights into the design of pH-universal electrocatalysts for carbon-neutral conversion of nitrate waste to ammonia.

Zhen Meng

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